DB2 'CASE' puzzle slqerr -811

Hi  everybody?
here is 2 CASE EXAMPLES:

WITH CASE_SUB(A,B) AS
 (VALUES('z','1'),
              ('z','2'),
             ('x','1'))
SELECT
CASE
  WHEN (SELECT COUNT(A) from case_sub WHERE A='z') = 1
  THEN 'A'
  ELSE 'B'
END AS CASE
FROM CASE_SUB;

CASE
----
B   
B   
B   

  3 record(s) selected.

2ND EXAMPLE:

WITH CASE_SUB(A,B) AS
 (VALUES('z','1'),
               ('z','2'),
              ('x','1'))
SELECT
CASE
  WHEN (SELECT COUNT(A) from case_sub WHERE A='z') = 1
THEN
  (select B from CASE_SUB WHERE A='z')
ELSE 'B'
END  AS CASE
FROM CASE_SUB;

sqlcode: -811 The result of a scalar fullselect, SELECT INTO statement, or
VALUES INTO statement is more than one row.
WHY ELSE 'B' is not executed?

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